The present invention relates to a method for selecting a codec for transmission of audio and/or video data between terminals through a packet switching connection, the terminals selecting the codec from a codec list, as well as a terminal for carrying out the method.
For transmitting audio and/or video data flows via packet-oriented transport protocols, such as, for example, IP (Internet protocol), a number of coding methods in the form of codecs is available. The latter differ regarding data rate, necessary computing power and achievable voice or picture quality. It is desirable to choose for each data connection the optimum codec, which guarantees the best possible quality without exceeding the available data rate.
For selecting the codec to be used, different possibilities are known, for example from ITU Recommendation H.245 (“Control Protocol for Multimedia Communication”, 1998) or IETF RFC 2327 (“SDP: Session Description Protocol”, April 1998). It is their common feature that the terminals possess in each case a list of supported codecs, from which one codec is selected. One possibility of negotiating consists in offering the codecs to the distant end, one after the other, in the order of the list. The first coding method accepted by the distant end will be used. Another possibility consists in one terminal transmitting its list to the distant end and the latter selecting, through comparison of the lists, the first codec suggested by both devices. In both variants, however, the available network capacity between the terminals involved is not taken into account.
Another possibility consists in differentiating different address ranges and setting a preferred codec or a codec list for each range. This codec or codec list will then be used for negotiating. In this way, a better adaptation of the coding method to the capacity available in the network can be achieved, this requires, however, a considerable configuration expenditure, both for the first putting into service of the terminal and for each major change in the network architecture.